Any older boat can have a wet core. Before purchasing, no matter the price of the boat, get it surveyed. This will limit the surprises you will have after taking ownership. I also reccommend not buying a boat right at the top of your budget. This will allow money to be left for repairs or new equipment. In the first year you will either need to fix something or will want to upgrade something after sailing for a while.
The Cal 28 MkI will probably be the most fun to sail. It is also a solidly built boat. I am partial to Pearsons. If you come across one I suggest giving it a serious look.